This is an improvement in the invention described in U.S. Pat. No. 5,248,566 issued Sep. 28, 1993, the disclosure of which is herein incorporated by reference and U.S. application Ser. No. 08/867,556 filed Jun. 2, 1997.
Fuel cells are being developed for use in both stationary form and in automotive propulsion systems as alternatives for the internal combustion engine in buses, vans, passenger cars and other vehicles. The major motivations for developing fuel cell powered vehicles are low pollution emissions, high fuel energy conversion efficiencies, superior acceleration, low noise and vibration and the possible use of coal or biomass derived alcohols rather than petroleum-base fuels. Although petroleum based fuels can also be used, the present invention is directed most specifically to systems for using hydrocarbons such as gasoline, diesel fuel or alcohols such as methanol as fuel.
The two most important operational requirements for a stand-alone fuel cell power system for a vehicle are the ability to start-up quickly and the ability to supply the necessary power and demand for the dynamically fluctuating load. The rapid start-up requirement is obvious.
A variety of hydrocarbons are possible fuels for use in transportation applications but most likely alcohols such as methanol are preferred fuels for use in fuel cells for transportation applications. Methanol is a commodity chemical that is manufactured from coal, natural gas and other feed stocks, while ethanol is often produced from grain. For use in a fuel cell, however, alcohols and hydrocarbons must first be converted (reformed) to a hydrogen rich gas mixture. The desired features for such a fuel reformer include rapid start-up, good dynamic response, fuel conversion, small size and weight, simple construction and operation and low cost.
Methanol has been used in steam reforming for providing a hydrogen rich gas stream for mobile combustion engines, see Koenig et al. U.S. Pat. No. 4,716,859, and water, as a reaction product from a fuel cell, has been recycled for use in steam reforming of methanol, see Baker U.S. Pat. No. 4,365,006. Steam reforming of methanol is endothermic and complicates, by its energy requirements, its use in a vehicle. Supplying the hydrogen rich gas on demand in an intermittent variable demanding environment is also a difficult requirement to meet and has been addressed by Ohsaki et al. U.S. Pat. No. 4,988,580 but this suggestion is not applicable to a small, mobile system. The catalytic exothermic partial oxidation-reforming of fuels to produce hydrogen-rich gas streams is known, see Rao U.S. Pat. No. 4,999,993. The use of a partial oxidation reformer had not been used in a vehicle to accomplish the purposes of this invention prior to the disclosure of the Kumar et al. '566 patent which is satisfactory for its intended purposes, but was based on theoretical considerations.
Converting hydrocarbon fuels to hydrogen can be done by steam reforming (reaction of the hydrocarbon with steam) or by partial oxidation (reaction with a substoichiometric amount of air). Steam reforming reactors are fairly bulky and are heat-transfer limited, while the partial oxidation reaction is more rapid and exothermic but less developed.
The parent of this application provided a catalyst for the exothermic partial oxidation reaction and was desirable since the endothermic steam reforming reaction required temperatures of about 1000.degree. C. At lower temperatures, the reactors can be smaller, can be made from less expensive materials like steels which are easier to fabricate and the product gas contains higher concentrations of hydrogen and less carbon monoxide, which is desirable. However, an appropriate catalyst for the endothermic steam reforming reaction has heretofore not been available, while the parent to this application disclosed a catalyst suitable for exothermic partial oxidation reaction. This invention is based on the surprising discovery that catalysts that are effective for the conversion of a wide range of hydrocarbons, including aliphatic, aromatics and others to hydrogen in the exothermic partial oxidation reaction are also effective in the endothermic steam reforming reaction.